Horizontal growth of crystal ribbons

ABSTRACT

A METHOD AND APPARATUS IS DESCRIBED FOR HORIZONATLLY PULLING CRYSTAL RIBBONS FROM A MELT PARALLEL TO THE MELT SURFACE. THE HEAT OF CRYSTALLIZATION IS SELECTIVELY REMOVED FROM ONE END OF A SEED CRYSTAL BY A HEAT ABSROBING MEANS PERPENDICULAR TO THE MELT SURFACE. MEANS ARE PROVIDED TO APPROPRIATELY SUPPRESS NET LOSS OF THE HEAT OF FUSION FROM THE BALANCE OF THE SEED IN CONTACT WITH THE MELT TO CONTROL THE THICKNESS OF THE RESULTANT RIBBON.

Sept. 18, 1973 c. E. BLEIL HORIZONTAL GROWTH OF CRYSTAL RIBBONS Original Filed Jan. 31,

United States Patent O US. Cl. 23--301 SP 5 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus is described for horizontally pulling crystal ribbons from a melt parallel to the melt surface. The heat of crystallization is selectively removed from one end of a seed crystal by a heat absorbing means perpendicular to the melt surface. Means are provided to appropriately suppress net loss of the heat of fusion from the balance of the seed in contact with the melt to control the thickness of the resultant ribbon.

RELATED PATENT APPLICATION This application is a division of US. patent application Ser. No. 795,561, entitled Horizontal Growth of Crystal Ribbons, filed Jan. 31, 1969, in the name of Carl E. Bleil, and assigned to the assignee of this application.

BACKGROUND OF THE INVENTION Thin wafers of monocrystalline semiconductive materials are generally produced from monocrystalline boules grown by the Czochralski technique. The preparation of the thin wafers from the large crystal boule, as is well known, is a costly and time-consuming technique, inherently wasting much of the boule. Consequently, much effort has been directed toward growing thin monocrystalline ribbons, that need only be scribed and broken to be used. In addition, where larger area wafers are desired, more slender boules have been grown, so that the boules need only be sliced to 'be used. However, most devices require small area wafers, and thin ribbon growth is much more alluring. Thin ribbons have been grown through elongate openings in a melt cover. In this latter technique the crystal is pulled perpendicular to the surface of the melt through a melt cover slot, so that the transverse cross section of the resultant ribbon conforms generally to the shape of the slot. However, satisfactory larger ribbon widths are extremely difficult to produce by means of this latter technique, and processing of the smaller ribbon widths is not commercially advantageous. Also, it has been proposed to horizontally produce flat ribbons by zone melting techniques. However, such techniques have not been wholly satisfactory for producing thin ribbons of high purity.

SUMMARY OF THE INVENTION It is, therefore, a principal object of this invention to provide a means for pulling relatively wide, high purity, thin crystalline ribbons directly from the surface of a melt. In particular, it is an object of this invention to provide a method and apparatus for pulling a crystalline ribbon horizontally from a melt. These and other objects of the invention are attained by placing a thin flat seed in contact with the surface of a melt, and withdrawing the heat of fusion generally perpendicularly from one end of the seed while maintaining the crystal-melt freezing isotherm coextensive with the desired ribbon thickness. This isotherm is maintained by suppressing the net loss of the heat of fusion from the balance of the seed. The seed is simultaneously pulled parallel to the surface of the melt at a rate commensurate with the rate at which the melt progressively solidifies on the end where heat is being withdrawn. In a preferred embodiment a heat sink is placed in contact with the end of the seed where growth is desired, and supplementary heaters and/or heat reflectors are used to suppress net loss of the heat of fusion from the balance of the seed. In one embodiment for growing semiconductor ribbons, the seed is wider than the contact surface on the heat sink, and the rate of pull is controlled so that the heat sink contacts only the growing seed, not the melt.

BRIEF DESCRIPTION OF THE DRAWING Other objects, features and advantages of the invention will become more apparent from the following description of preferred examples thereof and from the drawing, in which:

FIG. 1 shows an elevational view in partial section of the melt area of a Czochralski-type crystal growing apparatus modified with a heat sink for horizontal crystal growing in accordance with the invention;

FIG. 2 shows an enlarged fragmentary View of the meltseed-heat sink contact region of the apparatus shown in FIG. 1;

FIG. 3 shows an elevational view in partial section of a modification of the apparatus shown in FIG. 1, in which the heat sink includes a heat reflector to aid in controlling thickness of the crystalline ribbon grown;

FIG. 4 shows an enlarged fragmentary view of the melt-seed-heat sink contact region of the apparatus shown in FIG. 3;

FIG. 5 shows a transverse sectional view along the line 5-5 of FIG. 4; and

FIG. 6 shows an isometric view in partial section of another apparatus for horizontal crystal growing contemplated by the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As previously indicated, the principal feature of this invention lies in the selective removal of heat perpendicular to the surface of the melt at one end of a crystal seed on the surface of the melt, and the suppression of the net loss of the heat of fusion from the balance of the crystal seed. A heat sink can be used to selectively remove the heat of crystallization from the one end of the seed as shown in connection with FIGS. 1 through 5. On the other hand, the heat can be removed by simply allowing it to radiate to the ambient, as shown in FIG. 6. The end of the seed where the heat is lost becomes the growing end of the resultant ribbon. Growth is suppressed in the balance of the ribbon, so that its thickness is controlled. If thickness is not controlled at all, the ribbon may become so thick that it cannot even be pulled over the lip of the melt crucible. Growth can be suppressed by suppressing net loss of the heat of fusion. It can be suppressed with a special heater to replace any heat of fusion lost or with a heat reflector to suppress any actual loss of the heat of fusion, or both. Hence, by net heat loss I mean that if there is any actual loss of the heat of fusion it is replaced by a heat source other than the melt itself. Hence, this heater and/or the heat reflectors establish a freezing isotherm for the balance of the ribbon contacting the melt that is coextensive with the desired ribbon thickness. When the growing seed is pulled horizontally across the surface of the melt, it moves through the area where further crystal growth is controlled as desired.

The surface of the seed facing the heat absorber should be coplanar with the surface of the melt, at least adjacent the growing tip of the seed. By growing tip I refer to the transverse edge of the seed on its growing end where growth opposite the direction of pull occurs. The growing tip can be made coplanar with its adjacent melt surface,

by making the entire upper surface of the seed coplanar with the free surface of the melt. It can also be done by using a separate melt contact element, having a surface that is wetted by the melt, to lift the melt by surface tension forces to the desired level adjacent the seed growing tip. I prefer to use both principles to insure seed-melt coplanarity.

Moreover, the new growth on the seed may or may not adhere to the heat sink removing means. The crystal can be prevented from adhering to the contacting surface of the heat sink if there is no strong reaction between their contacting surfaces near the crystal melting point. For example, ice forming on a Teflon coated heat sink does not significantly adhere to it. If crystal-heat sink adherence is a problem, nucleation of the crystal must be induced away from the heat sink. This can readily be accomplished by starting with a seed that is wider (and, of course, much longer) than the seed contact area of the heat sink, generally centering the heat sink between the sides of the seed, spacing the heat sink back somewhat from the tip of the seed where growth is desired, and pulling the seed at an appropriate rate or adjusting the rate of nucleation to substantially maintain this spacing during crystal growth.

Of course, the usual precautions pertaining to the choice of crucible materials, ambient atmosphere, vibration-free environment, and other routine growth conditions should be observed for this invention as one would in the usual Czochralski crystal growing technique.

More detailed aspects of the invention can best be described by reference to the drawing. FIGS. 1 and 2 show the melt crucible area of a germanium crystal growing apparatus made in accordance with the invention. To better focus attention on the improvements this invention presents, the ancillary features normally incident to a Czochralski-type crystal growing apparatus, such as a housing and support means for producing a protective environment, heater power supplies, pulling apparatus and the like, are not shown. However, for example, the apparatus would involve a Pyrex furnace chamber, heating power supplies, means for producing and maintaining in it an atmosphere of 2 parts argon and 1 part hydrogen, etc.

In the crucible area of the grower, a rectangular graphite crucible rests on a crucible support 12 within an encircling radio frequency type heater 14 for heating the crucible to a temperature of about 960 C. The crucible 10 has an inner dimension of 2 /2 x 1% x 1% inches and has a /2 inch high graphite collar 16. It is slightly overfilled with a 540-gram melt 18 of germanium, so that the free melt surface 20 is held about 0.06 inch above the lip 22 on crucible collar 16.

One end of a monocrystalline seed 24, /2 inch wide and 0.03 inch in thickness, contacts the surface 20 of the melt. The major faces of the seed are parallel and in the (111) plane. The other end of the seed 24 is connected to a puller 26 for pulling it in the 211 direction. The seed is, therefore, pulled horizontally so that it, in effect, moves parallel over the surface of the melt. A special heater 28 is recessed in the surface of the melt under the seed adjacent the crucible wall over which the seed is pulled. It lies parallel that wall and transverse to the ribbon being formed to supply ribbon heat lost by radiation. It is spaced only about 0.010 inch below the ribbon and is adjusted to maintain the freezing isotherm in the system coextensive with the desired ribbon thickness. A spring loaded roller support 30 in the crucible collar 16 supports the ribbon as it is pulled over the lip 22 of the crucible.

The growing tip of the seed 24, opposite that end which is attached to the puller 26 contacts the surface 20 of the melt under a heat sink 32. The heat sink has a graphite cylindrical supporting shaft 34 one inch in diameter and a graphite cooling shoe 36. The supporting shaft 34 is cooled by cooling coils 38. The cooling shoe 36 is heated by an induction heater 40 to raise it to the temperature desired to support the particular rate of crystal growth.

4 The lower surface 42 of the shoe 36 is rectangular, being about one-half inch wide and one inch long. It is somewhat narrower than the seed and generally centered between the seed edges, as previously described. Also as previously described, it is spaced inward of the growing tip of the seed so that it does not contact the melt directly.

The shoe portion 36 of the heat sink also serves as a support for appendage 44, which contains an auxiliary heater 46. The heater 46 in appendage 44 aids in more precisely controlling melt temperature at the growing tip of the seed, the principal growth interface. The bottom end of the appendage has a projection 50 of a substance that is wetted especially well by the melt, such as quartz. The bottom surface of projection 50 is coplanar with the seed contact area of the heat sink and is a one-half inch square area, and spaced 0.25 inch from the contact surface of the heat sink. Being wetted by the melt, projection 50 serves to maintain the melt surface adjacent the growing tip of the seed coplanar with the top surface of the seed, even if that seed surface should vary somewhat from the level of the free surface of the melt. Since the seed floats on the surface of the melt, it is maintained coplanar therewith by the downward force of the heat sink. By mounting appendage 44 on the heat sink, the melt surface adjacent the growing tip of the seed can be maintained coplanar with the seed top surface even when the free melt surface is not.

A graphite bulb 52 having heater 54 is used as a melt level controller to make the process more continuous. The bulb 52 is progressively immersed more deeply into the melt as the crystal ribbon is formed to maintain the free surface of the melt substantially constant. It can be interconnected with the pulling apparatus (not shown) to maintain melt level automatically.

FIGS. 3 through 5 show a modification of the apparatus illustrated in connection with FIGS. 1 and 2. The modification involves the construction of the heat sink used to selectively remove heat from the seed and the nature of the special heating used to maintain the desired freezing isotherm. The heat sink 56 is FIGS. 3 through 5 is of a unitary construction and is wholly of graphite. Also, it contains a heat reflector section 48 to better control ribbon thickness and cross-sectional geometry.

Since the heat sink S6 in FIGS. 3-5 is of graphite, no special contact projection need be provided for the growth interface control appendage 58 attached to it. While a quartz surface might be wetted better, the graphite surface is adequate. However, since germanium will adhere to graphite when it solidifies, the growth interface on the end of the ribbon must be maintained between the appendage 58 and the main body portion of the heat sink, just as described in connection with FIGS. 1 and 2.

As in FIG. 1, a special heater 60 is used in the melt along the crucible wall 62 underneath the ribbon 64 so that the ribbon passes closely over it as it breaks contact with the melt. However, complementing heater 60 in controlling ribbon thickness is the heated roller 65 in the crucible wall 62. If desired, the heated roller 65 can be used as the principal source of heat to control the adjacent melt-crystal freezing isotherm. Complementing both the special heater 60 and the heated roller 65 is the heat reflector 48 in the adjacent end of the heat sink 56. The heat reflector can be of a laminate of a nickel sheet 66 and an outer quartz sheet 68. The heat shield suppresses radiation of heat from the non-growing end of the seed in contact with the melt. It not only suppresses thickening of the ribbon but also makes the bottom surface of the ribbon flatter. The heat reflector need not be made as a part of the heat sink, but the heat sink provides a convenient support. Using both the auxiliary heaters 60 and 65 under the ribbon and a heat shield over the ribbon, thickness can be more accurately controlled.

FIG. 6 illustrates a still further modification of the invention. FIG. 6 shows how heat can be selectively removed from one end 70 of the growing crystal 71 and heat loss suppressed in the balance of the crystal contacting the melt without even using a discrete heat sink element. The melt is contained in a graphite crucible 72 having a piston 74 for its bottom wall. Upward movement of the piston controls the level of the melt 76 within the crucible. Means for heating the crucible to a suitable temperature and for providing the usual crystal growing environment are not shown. The crucible is provided with a heat reflective cover 78 spaced from the upper surface of the melt to reflect back heat irradiated by the surface 8 of the melt. The cover can be of graphite having a heat reflective coating on its surface. A rectangular aperture 82 is provided in the surface of the cover of a width commensurate with the crystalline ribbon width desired. It is of a length (with respect to the axis of pull) sufficient to obtain the desired ribbon thickness. Heat is selectively removed from the growing end 70 of the ribbon 71 by permitting radiant heat loss from the growing end of the ribbon to the surrounding environment. Actual heat loss from the balance of the seed in contact with the melt is suppressed by means of the heat reflective metal cover. Net heat loss in the balance can be further suppressed if desired by means of a special heater 84 in the crucible wall over which the crystalline ribbon is pulled. Hence, analogous to the apparatus of FIGS. 3-5 a special heater and a heat shield are used. It may even be desired to supplement this by means of a special heater (not shown) within the melt itself adjacent the crucible wall over which the crystal ribbon is pulled, as in the apparatus shown in connection with FIGS. 3-5. The process can be conducted continuously by means of a automatic sensors 86 and 88 to determine melt level and temperature, respectively.

It is to be appreciated that although this invention has been described in connection with certain specific examples thereof, no limitation is intended thereby except as defined in the appended claims.

[[ claim:

1. The method of horizontally growing a fiat crystalline ribbon from a melt comprising the steps of heating a melt in a crucible, placing on the melt upper surface a crystalline seed having at least one broad flat upper surface with a growing end and an opposite end, maintaining said melt upper surface substantially coextensive and in contact with said seed undersurface and, at the initial growing end of said seed, in contact with the upper surface of said growing end, selectively vertically removing heat of fusion from the upper surface of said seed to selectively induce progressive melt solidfication on said growing end of said seed, establishing a temperature gradient in the melt perpendicular to said seed upper surface so as to provide a horizontal melt-seed freezing isotherm commensurate with the thickness of crystalline ribbon desired, suppressing net loss of the heat of fusion from areas of said seed in contact with the melt but removed from said growing end to suppress crystallization thereat and avoid excessive growth on the underside of the seed which would inhibit withdrawal of the seed over a lip of said crucible, and horizontally pulling the opposite end of said seed away from melt to pull said seed across said melt surface and over said crucible lip at a rate commensurate with said progressive solidification on said growing end and to progressively grow a flat crystalline ribbon from said melt.

2. The method as defined in claim 1 wherein the seed is monocrystalline, the seed and the melt are of a semiconductor selected from the class consisting of germanium and silicon, heat is selectively vertically removed from said growing end of said upper surface of said seed by conduction through a discrete heat sink member contacting the upper surface of said growing end inwardly of the fiat broad seed tip and sides, and during progressive solidification of the melt the seed is pulled at a rate to maintain the heat sink-seed contact area spaced inwardly from said tip and sides.

3. The method as defined in claim 2 wherein auxiliary means are used to selectively heat the surface of the melt adjacent the initial growing end of said seed to more accurately control melt temperature thereat and the net loss of the heat of fusion in areas of the seed removed from said growing end is suppressed with auxiliary heating means adjacent the seed.

4. The method as defined in claim 1 wherein the heat of fusion is selectively removed from said growing end of said seed by selectively permitting said growing end to radiate said heat to its environment and net loss of the heat of fusion from the balance of the seed is suppressed by reflecting any radiation emitted from the seed in areas removed from said growing end back to those areas.

5. The method of continuously horizontally growing a flat crystalline ribbon from a germanium melt comprising the steps of heating a germanium melt in a crucible, placing on the germanium melt upper surface a crystalline seed having at least one broad flat upper surface with a growing end, maintaining said melt upper surface substantially coextensive and in contact with said seed undersurface and, at the initial growing end of said seed, in contact with the upper surface of said growing end, contacting said seed upper surface at said growing end with a heat conductor, selectively vertically removing heat of fusion from said growing end of said seed by conduction to induce progressive melt solidification on said growing end, establishing a temperature gradient in the melt perpendicular to said seed broad area so as to provide a horizontal melt-seed freezing isotherm commensurate with the thickness of crystalline ribbon desired, suppressing net loss of heat of fusion from areas of said seed in contact with the melt but removed from said growing end to suppress crystallization thereat and avoid excessive growth on the underside of the seed which would inhibit withdrawal of the seed over a lip of said crucible, horizontally pulling said seed across said melt upper surface and over said crucible lip at a rate commensurate with said progressive solidification to progressively grow a flat crystalline ribbon from said melt, and maintaining the upper surface of said melt substantially constant as melt volume decreases during said progressive solidification on said seed.

References Cited UNITED STATES PATENTS 2,698,467 1/1955 Tarquinee et al. 23-301 2,907,715 10/1959 Cornelison 23-301 2,992,903 7/1961 Imber 23-301 3,031,275 4/1962 Shockley 23-301 3,370,927 2/1968 Faust, Jr. 23-301 3,378,350 4/1968 Sasaki 23-301 3,464,812 9/1969 Utech et al 23-301 3,494,745 2/ 1970 Herczog et al. 23301 NORMAN YUDKOFF, Primary Examiner R. T. FOSTER,'Assistant Examiner US. Cl. X.R. 23-273 SP 

